Method of making encapsulated resistors of high precision



METHOD OF MAKING ENCAPSULATED RESISTORS OF HIGH PRECISION Original FiledMarsh 28, 19611 INVENTOR Lu/ Tia $64 M .4 n-cmvevs United States Patent3,348,304 METHOD OF MAKING ENCAPSULATED RESISTORS OF HIGH PRECISIONLuigi Tassara, Milan, Italy, assiguor to Consolidated ElectronicsIndustries Corp., New York, N.Y., a corporation of Delaware Originalapplication Mar. 28, 1961, Ser. No. 98,861, now Patent No. 3,213,402.Divided and this application Dec. 9, 1964, Ser. No. 429,693

4 Claims. (Cl. 29-610) ABSTRACT on THE DISCLOSURE The invention isdirected to a method of making high precision encapsulated resistors byconnecting a plurality of resistors together and encapsulating them in abody having a hollow space to receive an additional resistor. After theinitial resistors have been encapsulated, they are measured, and acompensating resistor having the proper resistance value to make theoverall resistance correct is connected to the initial resistors andphysically placed in a hollow space and encapsulated therein. Anydeviation in the resistance of the final resistor due to theencapsulation procedure will then have only a minor effect on theoverall resistance of the complete multi-part resistor.

This application is a division of my co-pending application Ser. No.98,861, now Patent 3,213,402.

The present invention relates to encapsulated resistors of highprecision, for example of the order of .1%, and to a method for makingthe same and retaining the high precision in spite of the change inresistance produced by the encapsulation process. The invention isparticularly adaptable to resistors of high ohmic value.

A resistor constructed according to the invention comprises a pluralityof individual resistors not previously encapsulated connected togetherend to end and arranged so as to be physically parallel to each otherwithin a container. The location of the resistors within the containeris such that one end of each of the resistors points in the samedirection toward the open mouth of the container to extend above thesurface of encapsulating material poured into the container. Alsolocated within the container is a region dammed off from the remainderand of sufficient size to receive an additional resistor. The ohmicvalue of the series-connected resistors is predetermined to be slightlyless than the desired ohmic value of the finished resistor, and afterthe encapsulating material has been poured around the series-connectedresistors and allowed to set, the resultant resistance is measured andan additional resistor sufiicient to bring the total series resistanceup to the desired value is connected to the other resistors and isinserted into the dammed-01f section. Thereafter, this section is alsofilled with encapsulating material and additional encapsulating materialis added to cover the open ends of all of the resistors.

The composite resistor may be formed so as to be easily connected inseries in similar composite resistors by providing a terminal at eachend thereof, one of the terminals comprising an internally threaded nutand the other terminal comprising an externally threaded bolt.

The invention will be further described in connection with the drawings,in which:

FIG. 1 shows a cross-section of a high precision resistor constructed inaccordance with this invention;

FIG. 2 is a schematic representation of the resistor of FIG. 1 at anintermediate stage of manufacture;

FIG. 3 is a plan view of the resistor of FIG. 2; and

3,348,304 Patented Oct. 24, 1967 FIG. 4 is a schematic representation ofthe electrical connections and measuring points of the resistor of FIG.

The resistor in FIG. 1 is enclosed within hollow container 1, preferablyof synthetic resin, which is open at one end. A plurality of resistors2, only one of which appears in the drawing, is located within thecontainer 1 and physically positioned parallel to each other pointing,respectively, at the closed'end 3 and the open end 4 of the container 1.The resistors 2 may comprise any suitable form of resistor, such as aninsulating body coated with a conductive layer of metal or othermaterial. The resistors are connected as shown in FIG. 2 so as to beelectrically in series, with alternate junctions between resistorsadjacent the closed end 3 and the open end 4 of the container.

An opening 6 is provided in the closed end 3 of the container and aconductive metal bushing 7 having a locking flange 8 is placed in theopening to fill it up and serve as a closure therefor. The bushing isinternally threaded, as indicated by reference numeral 9, and has acylindrical appendage 10 extending into the container. The externalsurface of the appendage 10 is knurled or otherwise roughened to receiveand to hold fast to one end of a tube 11 of insulating material. Oneterminal or lead 12 at one end of the series-connected resistors 2 iswound around and conductively joined to the appendage 10. A machinescrew 13 together with a washer 14 may be used as an external fasteningmeans to make electrical connection to the outside of the compositeresistor in order to connect with the terminal 12 within the container1.

The tube 11 serves to darn off a region within the container 1 when thelatter is placed in the position shown in FIG. 2 and is partially filledwith an encapsulating material 5 in liquid form. A compensating resistor15 is chosen and placed within the dammed-off region and is connected bymeans of a lead 16 to the opposite end of the series-connected resistorsfrom the lead 12. The selection of resistor 15 will be described ingreater detail hereinafter.

The container 1 is thereafter filled with the encapsulating materialwhich is allowed to set. Subsequently, a hole 17 is formed in thesurface of the material 5 and a hollow bushing 18 having an extension 19is forced into the hole 17. Prefereably, this bushing is coaxial withthe bushing 7 at the other end of the container 1. The bushing 18 has alocking collar 20 to prevent it from turning and to increase thefriction on the surface ofthe encapsulating material 5 and the outer endof the bushing is threaded, as indicated by reference numeral 21..Preferably, this thread matches the internal thread of the bushing 7 sothat similar composite resistors can be joined end to end, as indicatedby the container 1 and the container 1a. The bushing 18 has an axialhole 22 through which extends a lead 23 from the compensating resistor15. The protruding end of the lead may be cut off and welded to thebushing, as indicated by reference character 24, so as to seal oif theassembly.

The process of forming the composite resistor just described will now beconsidered in greater detail. Reference has already been made to thefact that alternate connections between the resistors 2 are adjacent theclosed end 3 and the open end 4 of the container 1. If the resistors arevery numerous, they may be arranged in a spiral, as indicated in FIG. 3.

The resistors 2 are chosen so that when connected in series their totalohmic value will be slightly less than the desired value. After theresistors have been introduced into the container 1 and after connectionhas been made between the lead 12 and the appendage 10 and the tube 11has been attached to the appendage 10, the container 1 is placed in theposition as shown in FIG. 2 and filled to the level 25 which, it will benoted, is lower than the open end of the tube 11. Therefore, the latterserves as a dam and prevents the encapsulating material, which may be anepoxy resin, for example, from spilling over into the region within thetube 11. The level 25 is such as to immerse completely the resistiveelement portion of each of the resistors 2, leaving only the conductiveterminals 26 between resistors above the level of the encapsulatingmaterial. A measurement may then be made of the total resistance betweenthe lead 12 and the lead 16 at the ends of the series-connectedresistors. The compensating resistor 15 is then selected to have a valueequal to the difference between the desired value of the compositeresistor and the total value of the resistors 2. If a composite resistorof high ohmic value is to be made, it may be necessary to measure theresistance of one pair of resistors 2 at a time. Such measurements maybe made at the points indicated by the reference letters A, B, C and D,as shown in FIG. 4, and it is for this reason that it is desirable tohave the terminals 26 remain above the level 25 of the encapsulatingmaterial 5.

While each of the resistors 2 could be measured to any desired accuracybefore being encapsulated, the process of encapsulation produces achange in the resistance, and this change cannot be predicted precisely.The change may not be large, for example it may only be approximately1%. However, if it is desired to produce a-composite resistor to anaccuracy of .l%, clearly a change of 1% would be intolerable,particularly where the change would vary from one composite resistor tothe next.

After the compensating resistor 15 has been selected, one of its leadsis connected to the lead 16 of the resistors 2 and the compensatingresistor is placed within the tube 11. Thereafter, more encapsulatingmaterial is added to bring the level up to the open end 4 of thecontainer 1. In so doing, encapsulating material spills over inside thetube 11 and encases the compensating resistor 15. This changes the valueof resistance of the compensating resistor, but since the totalresistance of the compensating resistor is only a small fraction of thetotal resistance of the composite resistor, any change in its value isrelatively negligible. The change in value of the resistance of thecomposite resistor due to a change in value of the resistance of thecompensating resistor is given by the equation a b=c, where c is thechange in resistance of the composite resistor, a is the fraction oftotal resistance of the composite resistor represented by thecompensating resistor and b is the change in resistance of thecompensating resistor. To be specific, if the change in value of thecomposite resistor given by the letter c must be less than .1% and ifthe change in resistance of the compensating resistor is known to be notgreater than 2% when it is encapsulated, then the compensating resistormust have a resistance no greater than 5% of the total value of thecomposite resistor.

Although this invention has been described in terms of a specificembodiment, it will be understood that modifications may be made thereinwithout departing from the scope of the invention as measured by thefollowing claims.

I claim:

1. The method of making a precision composite resistor comprising thesteps of: connecting a plurality of individual resistors into atwo-terminal network; placing said network inside a container having aninterior wall separating one portion thereof from the remainder, saidnetwork being located in said remainder of said container; partiallyfilling said remainder with encapsulating material; measuring theresistance of said network; connecting a compensating resistor theretoto adjust the overall resistance to a desired value; placing saidcompensating resistor Within said one portion; and subsequently fillingsaid container and said one portion with encapsulating material.

2. The method of making a precision composite resistor comprising thesteps of: connecting a plurality of individual resistors in series in atwo-terminal network; placing said network inside a container having aninterior wall separating one portion thereof from the re mainder, saidnetwork being located in said remainder with said resistors extending inthe same direction as said interior wall; partially filling saidremainder with encapsulating material to cover said resistorssubstantially entirely; measuring the resistance of said network;connecting in series with said network a compensating resistor having aresistance equal to the difference between a desired value and the valueof resistance of said network; placing said compensating resistor withinsaid one portion; and subsequently filling said container and said oneportion with encapsulating material.

3. The method of making a precision composite resistor comprising thesteps of: connecting a plurality of individual resistors in series in atwo-terminal network; placing said network inside a container having aninterior wall extending upwardly from the bottom of said container andseparating one portion of said container from the remainder; saidnetwork being located in said remainder with said resistors extendingsubstantially parallel to said wall, said resistors being shorter thansaid wall; partially filling said remainder with encapsulating materialto cover said resistors substantially entirely except for the uppermostterminals thereof; measuring the resistance of said network; connectingin series with said network a compensating resistor having a resistancesubstantially equal in value to the difference between a desiredresistance of said composite resistor and the measured value of saidnetwork; lacing said compensating resistor within said one portion; andsubsequently filling said container and said one portion withencapsulating material.

4. The method of claim 3 in which the measurement of resistance of saidnetwork is accomplished by measuring the resistance between electricallyadjacent ones of said uppermost terminals.

References Cited UNITED STATES PATENTS 7/1914 Smith 338-77 X 2/1925Dominguez 338-319

1. THE METHOD OF MAKING A PRECISION COMPOSITE RESISTOR COMPRISING THESTEPS OF: CONNECTING A PLURALITY OF INDIVIDUAL RESISTORS INTO ATWO-TERMINAL NETWORK; PLACING SAID NETWORK INSIDE A CONTAINER HAVING ANINTERIOR WALL SEPARATING ONE PORTION THEREOF FROM THE REMAINDER, SAIDNETWORK BEING LOCATED IN SAID REMAINDER OF SAID CONTAINER; PARTIALLYFILLING SAID REMAINDER WITH ENCAPSULATING MATERIAL; MEASURING THERESISTANCE OF SAID NETWORK; CONNECTING A COMPENSATING RESISTOR THERETOADJUST THE OVERALL RESISTANCE TO A DESIRED VALUE; PLACING SAIDCOMPENSATING RESISTOR WITHIN SAID ONE PORTION; AND SUBSEQUENTLY FILLKINGSAID CONTAINER AND SAID ONE PORTION WITH ENCAPSULATING MATERIAL.